I have a 150 gallon with a Porcupine Puffer, Harelquin Tusk, Mombassa Lion, Wartskin Angler, and a dwarf golden eel.

I don't want to get any more fish because at this point there is enough room that everyone gets along (the eel, angler and lion don't move a whole lot so they don't take up much real estate).

I was looking into getting some larger feather dusters, but I'm not sure if the puffer and tusk will eat them. I have small feather dusters on the live rock and on some snails and no one bothers them. If I get a bigger duster, will that attract the tusk and puffer?

Wish I seen this earlier I could have told you it wouldn't work. That was one of my first mistakes when I started my salt water tank. If the puffer just got the feather dusters crown it will grow back and you probably won't see any sign of life from it for about a month. If you pick it up and gently squeeze the tube you might feel it hit the side and that's a good way to determine whether or not it's still alive